Messages in this thread | | | Date | Thu, 5 Feb 2009 20:10:17 +0100 | From | Ingo Molnar <> | Subject | Re: pud_bad vs pud_bad |
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* Jeremy Fitzhardinge <jeremy@goop.org> wrote:
> Ingo Molnar wrote: >> * Jeremy Fitzhardinge <jeremy@goop.org> wrote: >> >> >>> I'm looking at unifying the 32 and 64-bit versions of pud_bad. >>> >>> 32-bits defines it as: >>> >>> static inline int pud_bad(pud_t pud) >>> { >>> return (pud_val(pud) & ~(PTE_PFN_MASK | _KERNPG_TABLE | _PAGE_USER)) != 0; >>> } >>> >>> and 64 as: >>> >>> static inline int pud_bad(pud_t pud) >>> { >>> return (pud_val(pud) & ~(PTE_PFN_MASK | _PAGE_USER)) != _KERNPG_TABLE; >>> } >>> >>> >>> I'm inclined to go with the 64-bit version, but I'm wondering if >>> there's something subtle I'm missing here. >>> >> >> Why go with the 64-bit version? The 32-bit check looks more compact and >> should result in smaller code. >> > > Well, its stricter. But I don't really understand what condition its > actually testing for.
Well it tests: "beyond the bits covered by PTE_PFN|_PAGE_USER, the rest must only be _KERNPG_TABLE".
The _KERNPG_TABLE bits are disjunct from PTE_PFN|_PAGE_USER bits, so this makes sense.
But the 32-bit check does the exact same thing but via a single binary operation: it checks whether any bits outside of those bits are zero - just via a simpler test that compiles to more compact code.
So i'd go with the 32-bit version. (unless there are some sign-extension complications i'm missing - but i think we got rid of those already.)
Ingo
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